PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 569 



Aporhyolyte. Granite. Conglomerate.] 

 Taconyte. 



No. 850. APORHYOLYTE (?) 



" Fine, crypto-crystalline form of the red rock at Duluth; of a reddish-brown color; the same as No. 42, 

 but fresher, and less granular." 



Kef. Annual Report, xii, page 40. 



Meg. A fine-grained, brownish-red rock, containing some small, red, porphyritic 

 feldspars and areas of a dark mineral. 



Mic. The section shows a groundmass of red color, which under crossed nicols 

 breaks up into the " patchy " areas so characteristic of these devitrified red rocks. 

 The patchy nature is due to areas of poikilitic quartz, which are full of small grains 

 of much altered and veddeued feldspar. Magnetite is common and there is also some 

 chlorite. There are a few small porphyritic feldspars, usually considerably altered. 

 Some of these are of plagioclase. A few small quartz grains also occur. One section. 



Age. Cabotian. 



Remark. This rock is similar to Nos. 42 and 45. These rocks are called apor- 

 hyolytes, but at the same time it is seen that they may be of a more basic nature 

 than are the rhyolytes. u. s. G. 



No. 851. GRANITE. 



Courtland, Nicollet county, opposite New Ulm. 

 Kef. Annual Report, xiii, page 40. 



Meg. A coarse-grained granite, composed of much reddish feldspar, less quartz, 

 and some black mineral, which now seems to be largely chlorite. There are a few 

 large, red, porphyritic feldspars. 



Mic. The section is very thick, and is made up largely of feldspar much stained 

 by hematite. The feldspar appears to be orthoclase, microcline and acid plagioclase. 

 Some chlorite is present, as is also ilmenite largely altered to gray leucoxene. 



One section. 



Age. Archean. u. s. G. 



No. 852. CONGLOMERATE. 



Courtland, Nicollet county, opposite New Ulm. 

 Ref. Annual Report, xiii, page 40. 



Meg. A rough, coarse-grained, quartz conglomerate with a red cement. 



Mic. The section is composed mainly of quartz pebbles in a sparse hematite 

 cement. The quartz is full of dust-like inclusions. Some of the quartz pebbles are 

 made up of several grains which show undulatory extinction and even granulation. 



One thick section. 



Age. Potsdam. u. s. G. 



No. 852A. TACONYTE. 



Pebble from No. 852. 



Ref. Annual Report, xiii, page 40. 



Meg. A reddish, compact rock, resembling a very fine-grained quartzyte. 



